The HIV serotyping effort is the largest effort of its kind mounted by the WHO. The Virus Biology Unit (VBS), LTCB, NCI provided pilot data on a log-reduction assay to assess viral neutralization. User-friendly software, the ID-50 program, running on IBM and Macintosh platforms was improved to permit virologists to calculate infectious- doses 50% (ID-50s) or to assess neutralization. After comparing pilot data from several different assays, the WHO team adopted the viral log-reduction assay as a standard for assessing HIV neutralization. The AIDS Clinical Trial Group also accepted the same assay as a standard. The ID-50 program is now in general distribution.